A general process of producing grain oriented electrical steel sheet involves: preparing a steel slab with a predetermined chemical composition; subjecting the steel slab to hot rolling and cold rolling to form a steel sheet; then subjecting the steel sheet to decarburization annealing; and subjecting the steel sheet to subsequent final annealing for secondary recrystallization. Secondary recrystallization occurs during the final annealing among these process steps to generate coarse grains with their easy magnetization axes aligned in the rolling direction, with the result that excellent magnetic properties can be obtained. Since this final annealing is performed on a coiled steel sheet over a long period of time, it is a common practice to apply to the steel sheet prior to the final annealing, an annealing separator mainly composed of magnesia, the annealing separator being applied as a slurry obtained by suspending the annealing separator with water to prevent sticking of inner and outer wraps of the coiled steel sheet.
In addition to serving as such an annealing separator, the magnesia also serves to react with an oxide layer mainly composed of SiO2, which layer is formed on a surface of the steel sheet during the decarburization annealing (primary recrystallization annealing) prior to the final annealing, to thereby form a forsterite (Mg2SiO4) film on the surface. It is very difficult to form a uniform forsterite film by coil annealing, and various proposals have been made to this end.
For example, JP 54-014566 B2 proposes a method of forming a uniform film, in which magnesia containing, in an amount of 1% to 20%, the particles passing through a 100-mesh sieve, but not through a 325-mesh sieve (44 μm to 150 μm) is used as an annealing separator to prevent sticking of wraps of a coiled steel sheet and improve the gas flowability in the coil.
We observed, however, in JP 54-014566 B2, the following problems: magnesia containing, in an amount 1% to 20%, the particles passing through a 100-mesh sieve, but not through a 325-mesh sieve (44 μm to 150 μm) is indeed very effective in forming a uniform forsterite film, but may cause surface roughness due to local projections formed on a surface of the forsterite film. This surface roughness also causes a reduction in the stacking factor in stacking products, as well as film defects due to dropping of the aforementioned projections.
It could therefore be helpful to provide an annealing separator for a grain oriented electrical steel sheet which does not inhibit the flowability of an atmospheric gas during the final annealing of the coil-shaped product and can prevent the occurrence of surface roughness.